1. Field of the Invention
This invention relates to optical transmission systems and more specifically to a fiber optic transmission system capable of carrying a broadband signal over single mode optical fiber with high optical launch power.
2. Technical Background
Economical distribution of broadband signal content (such as multi-channel cable TV) over a single mode optical fiber system requires use of high optical signal powers. High optical signal powers enable the splitting of the optical signal for distribution over multiple fiber paths or alternatively transmission of signals over a single fiber path with a large link loss, as is typical of long distance transmissions. The availability of efficient erbium doped fiber amplifiers (EDFAs) operating in the 1550 nm wavelength region, where standard telecommunication fiber exhibits its minimum attenuation, has motivated the development of broadband transmitters compatible with the gain bandwidth of EDFAs. However, standard telecommunication single mode fiber (e.g. Corning SMF-28® fiber) exhibits dispersion in the 1550 nm region, which precludes the use of directly modulated distributed feedback lasers (DFBs) as a transmitter for cable television or high bit-rate digital signals. Instead, a typical transmitter operating at 1550 nm includes a narrow line width, externally modulated continuous wave (cw) DFB laser. The DFB laser light beam carries no information-bearing signals until the external modulation acts on the laser light beam to impress the information-bearing signals thereon. Here, “light” is not restricted to the visible spectrum. The optical power is amplified by an EDFA which is downstream from the external modulator. Thus the information bearing light signal enters the fiber optic span with an optical signal power determined by the saturated output power of the EDFA. Commercially available EDFAs offer saturated output powers exceeding 20 dBm.
As is well known, stimulated Brillouin scattering (SBS) is a nonlinear optical effect that poses a significant restriction to the amount of narrow-linewidth optical power that can be launched into a long length of single mode optical fiber. For a given length of single-mode fiber with a given attenuation coefficient at the chosen optical wavelength, there is an optical-linewidth-dependent threshold power below which appreciable SBS does not occur. For standard commercially available telecommunication fiber operating at 1550 nm, the SBS threshold for an optical source (laser) with an optical linewidth less than 10 MHz is less than 7 dBm for a fiber optic link of approximately 50 kilometer length.
Previously, in order to launch high optical signal powers in the 1550 nm wavelength region for transmission of broadband signals such as cable television over long fiber distances, one must suppress SBS. SBS degrades Carrier to Noise Ratio (CNR), Composite Second Order (CSO) and Composite Triple Beat (CTB) and induces power-dependent nonlinear attenuation in an optical fiber link, thereby also reducing the received optical power.